1. Field of the Invention
The present invention relates to an improved method of impregnating electrolytic capacitor stacks or wound rolls with a polymer electrolyte, such as a hydroxyethylmethacrylate (HEMA) or hydroxyethylacrylate (HEA) based polymer electrolyte, to render them suitable for use in electrolytic capacitors, and to such electrolytic capacitors.
2. Related Art
Conventionally, an electrolytic capacitor includes an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. Typically, the electrolytic or ion-producing component of the electrolyte is a salt that is dissolved in the solvent. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, composing a planar, layered, stack structure of electrode materials with separators interposed therebetween.
Typically, the separator is impregnated with a polymer electrolyte, such as a HEMA based polymer electrolyte. In known processes for impregnating electrolytic capacitor stacks or wound rolls with a polymer electrolyte, such as a HEMA based polymer electrolyte, a polymerization initiator is mixed with the electrolyte prior to impregnation. For example, U.S. Pat. No. 5,628,801 discloses an electrolytic capacitor where a separator impregnated with an elastomeric solid electrolyte is utilized in the dual capacity of electrolyte and adhesive material to hold together the anode and cathode plates of the capacitor. The preferred electrolyte consists of: 17.5 parts of hydroxyethylmethacrylate, 32.5 parts ethylene glycol, 7.0 parts ammonium adipate, 6.7 parts ammonium glutarate, 0.45 parts tetraethyleneglycodiacrylate, and 2.2 parts of initiator solution. The capacitor assembly is impregnated with this polymerizable liquid electrolyte/adhesive and then heated to approximately 55.degree. C. for at least 2 hours, but preferably 24 hours to cure the electrolyte/adhesive.
Similarly, U.S. Pat. No. 5,748,439 discloses an electrolytic capacitor having interposed between the electrically conductive anode and cathode layers thereof a spacer comprised of a mechanical separator means such as kraft paper impregnated with a crosslinked elastomeric electrolyte. The electrolyte is preferably made up as a liquid prepolymer electrolyte mixture prior to impregnation into the capacitor element and the polymer is preferably formed in situ thereafter from the prepolymer mixture. The mixture is preferably made up by first dissolving a salt into a liquid plasticizer component by stirring at elevated temperatures, cooling the mixture to room temperature, and then adding to the mixture a monomer corresponding to the desired polymer and a crosslinking agent, as well as a polymerization initiator.
However, mixing the initiator with the electrolyte prior to impregnation, as disclosed in both of these patents, has the disadvantage of shortening the working life of the electrolyte, and making impregnation into fine structures and between anode plates difficult, due to the inherent higher viscosity at room temperature.
Heating the electrolyte to reduce viscosity, a common practice in the industry, only serves to hasten the curing of the polymer and thus defeats the intended purpose.